1. Field of the Invention
The present invention relates to an image forming process and, particularly, to a dry type process for recording images using a photo-polymerizable material.
2. Description of the Prior Art
Heretofore, as photoresist materials for producing printed circuits, solvent development type recording materials such as a photosensitive liquid comprising a combination of a bichromate and glue or polyvinyl alcohol, a photosensitive liquid comprising polyvinyl cinnamate and a sensitizing agent as main ingredients or a photosensitive liquid comprising natural rubber or cyclized rubber and a cross-linking agent as main ingredients have been commonly used.
There are also known photoresist materials which comprise a photosensitive material inserted between two plastic films. In such photoresist materials, one of the plastic films is stripped off to expose the photosensitive material upon use. After being superposed on a desired substrate for printed circuits, (e.g., a copperized plate), the photosensitive layer is image-wise exposed to light through the remaining plastic film. The remaining plastic film is then removed and the photosensitive layer developed by a suitable developer such as an organic solvent or an aqueous alkali solution, etc. The photosensitive layer hardened by exposure remains on the substrate while non-exposed areas are removed by dissolution in a developer to form a resist pattern for printed circuit manufacture. However, processing the above described solvent development type recording materials is complicated as a developer must be used. The developers commonly used result in waste liquids which pose a danger of environmental pollution.
Recently, dry development type image recording materials and processes have been proposed as an alternative to the above described solvent development type recording materials, e.g., in Japanese Patent Publications 9663/63 and 22901/68, Japanese Patent Application (OPI) 7728/72, Japanese Patent Publication 43126/73 and Japanese Patent Application (OPI) 33623/72, processes are described for forming images utilizing a photosensitive layer where the degree of adhesion of exposed areas and unexposed areas to the support or to the substrate are reversed, for example, the adhesion of the photosensitive layer to the substrate is high in exposed areas while the adhesion of the photosensitive layer to the support is low in the unexposed portion. This process generally comprises applying a photo-polymerizable composition comprising an addition polymerizable monomer, a photo-polymerization initiator and a binder as a photosensitive layer to a transparent support such as a thin transparent plastic film, laminating the photosensitive layer on a transparent or opaque substrate such as a metal plate, a plastic plate or film, or paper, etc., image-wise exposing to light through the transparent support using an original and stripping off the support to leave either the exposed portion or the unexposed portion of the photosensitive layer on the substrate or the support, by which positive images are formed on one of the substrate and the support and negative images are formed on the other. According to this recording process, an image resist pattern can be formed merely by stripping off the support, i.e., development in a liquid is omitted. Accordingly, the production of the printed circuits becomes more economical and simpler from the production viewpoint. Further, there is no danger of environmental pollution as waste liquid is not generated.
As the addition polymerizable monomers used in the phososensitive layer of such dry (stripping) type developable recording materials, heretofore acrylic acid esters or methacrylic acid esters prepared from acrylic acid or methacrylic acid and polyhydroxy compound, such as pentaerythritol triacrylate, pentaerythritol trimethacrylate, trimethylolpropane triacrylate or diethyleneglycol diacrylate, as described in Japanese Patent Publication 15932/66 and Japanese Patent Application (OPI) 7728/72, have been used.
On the other hand, though may high molecular weight materials are described in the above patent publications as the binder constituent of the photosensitive layer, only a few limited materials such as chlorinated rubber can be used as a main ingredient of the binder when the substrate is composed of metal. According to studies by the inventor, it has been confirmed it is necessary to use chlorinated rubber as a main ingredient in order to obtain good images by stripping development. Further, it has been found that chlorinated polyolefins are suitable as a binder to obtain good images by stripping development in the case of using the above described monomers (see Japanese Patent Application 97049/73).
In the production of a resist pattern on a substrate, there are cases where all of the resist pattern is intimately adhered to a substrate such as a copper plate superposed on a plastic plate. On the other hand, in the case of producing a printed circuit using a substrate having "through holes", where both the surface of the substrate and the inside faces of the through-holes are covered with a metal layer so that circuits formed on both surfaces of the substrate are connected by electrical conduction through the inside faces of the through holes, it is necessary to form a resist film over the through holes without contacting the resist film with the metal surface on the through holes (this is called tenting).
Tenting is necessary to prevent etching of the metal layer on the inside face of the through holes when printed circuits are formed by etching. In the case of such tenting, it is required that the resist film formed adhere closely to the substrate at only very small areas around the through holes while other portions of the resist film be supported by the cohesive force of the film over the through holes, and that such a film have a strength sufficient to endure the mechanical shock of the spraying pressure in a spray type etching process.
It has become clear, as a result of studies by the inventor, that though stripping development type photoresist films prepared using the above described esters of acrylic acid or methacrylic acid and polyhydroxy compounds as a monomer and the above described chlorinated rubber or chlorinated polyolefins as a main ingredient of the binder have sufficient resistance where the whole resist pattern is intimately adhered to the substrate, they do not have strength sufficient to withstand spraying pressure at etching when they are used for tenting.